1. Field of the Invention
This invention relates to a control method for injection molding machines in which control is made such that a final control amount of an object subjected to control which is controlled by an actuator becomes a set value of a previously set molding condition.
2. Description of the Background Art
Generally, in an injection molding machine, for example, as disclosed in Japanese Patent Laid-open No. 4(1992)-131217, feedback control is made such that an injection speed (final control amount) of a screw (object subjected to control) which is controlled by an injection cylinder (actuator) becomes a set value of a previously set molding condition.
Such a control method will be explained with reference to a feedback control system as shown in FIG. 5. At first, a position of an injection cylinder (screw) 62 is detected by a position detector 61, and this detected value Dod is converted into speed information (detected value Vod) by a speed converting unit 63. A set value Vos of the injection speed as a molding condition and the detected value Vod are subjected to subtraction processing by a subracting unit 64. An obtained deviation value Voc is subjected to PID compensation by a PID compensating unit 65. Thereafter it is given to a servo valve 66. The servo valve 66 is used to make feedback control of the action of the injection cylinder 62. Incidentally, such control is executed in the same manner even in the case of an analog signal processing system or a digital signal processing system.
However, with respect to such a conventional control method, the control is made using the feedback control system including the PID compensating unit 62, so that in the case of the analog signal processing system, adjustment for determining a PID constant and the like is not easy. The adjustment is inevitably performed to make selection to moderation of whole characteristics. Therefore, it becomes difficult to set good characteristics for all ranges, which consequently results in such a weakness that an individual error and a mechanical error are apt to occur although it takes a considerable time. This, problem become conspicuous when a proportional servo valve in which the flow amount characteristic does not have a linear characteristic is used as the servo valve 66. On the other hand, in the case of the digital signal processing system (digital servo system) in which a control algorithm is executed and processed by a microcomputer, there has been such a weakness that it is difficult to execute the linear control in which calculation equations become linear. In addition, in the control system in which such PID compensation is made, as in an injection speed characteristic curve Fv, as shown in FIG. 3 by a two-dotted line, there has been such a weakness that the response property is bad, and highly accurate and highly stable control cannot be made due to occurrence of over shoot (or under shoot).